A stent is a mesh ‘tube’ inserted into a natural passage/conduit in the body to remove or counteract a disease-induced, localized flow constriction. The term may also refer to a tube used to temporarily hold such a natural conduit open to allow access for surgery.
Most of the time, stents are used to treat conditions that result when arteries become narrow or blocked. When a stent is placed into the body, the procedure is called stenting. A stent is placed in an artery as part of a procedure called angioplasty. Angioplasty restores blood flow through narrow or blocked arteries. A stent helps support the inner wall of the artery after angioplasty.
Stents are generally tubular devices for insertion into body lumens. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent is biased so as to expand upon release from the delivery catheter. Some stents may be characterized as hybrid stents which have some characteristics of both self-expandable and balloon expandable stents. Almost all stents used in the treatment of coronary atherosclerosis are balloon expandable. Self-expandable stents are generally used in larger blood vessel in the limbs and periphery.
There are different kinds of stents. Stents usually are made of metal mesh of various metals and alloy combinations. Some stents are a plastic mesh-like material, and some stents are a combination of metal and synthetic lining material (for example PTFE-Polytetrafluoroethylene) and are called stent grafts and are used in larger arteries.
An intraluminal coronary artery stent is a small, balloon-expandable, metal mesh tube that is placed inside a coronary artery to prevent the artery from re-closing. The metal portion of the structure is called a strut and the open portion of the mesh between struts is called a cell. Re-narrowing of arteries at the site of stent deployment has been addressed with medicine coated stents, called drug-eluting stents. Like other coronary artery stents, it is left permanently in the artery.
Stents may be constructed from a variety of materials such as stainless steel, Elgiloy®, nitinol, shape memory polymers, etc. Stents may also be formed in a variety of manners as well. For example a stent may be formed by etching or cutting the stent pattern from a tube or section of stent material; a sheet of stent material may be cut or etched according to a desired stent pattern whereupon the sheet may be rolled or otherwise formed into the desired tubular or bifurcated tubular shape of the stent; one or more wires or ribbons of stent material may be braided or otherwise formed into a desired shape and pattern.
Repair of coronary vessels that are diseased at a bifurcation is particularly challenging since the stent must be precisely positioned, provide adequate coverage of the disease, provide access to any diseased area located distal to the bifurcation, and maintain vessel patency in order to allow adequate blood flow to reach the myocardium.
Currently employed techniques have results that are less favorable than stenting results for lesions that do not involve bifurcations. The most commonly employed technique is to introduce guide-wires into the main blood vessel (parent-vessel) and the side branch (daughter-vessel). The ostium of the daughter-vessel is treated with balloon angioplasty and then a stent is deployed in the parent-vessel as if there was no bifurcation involvement. The hope is to have a stent cell line up with the ostium of the daughter-vessel resulting in unobstructed flow. However, in reality this is not always the case and stent struts usually are left in the ostium increasing the risk of acute and subacute stent thrombosis. Also promotion of neointimal growth onto the unopposed struts can result in renarrowing. A daughter-vessel is in effect “jailed” by the stent and blood flow can continue to be compromised results in inadequate treatment.
Another phenomenon that is of concern in treatment of bifurcations is plaque shift. When a balloon or a stent is deployed in an artery, the plaque is compressed against the vessel wall. However, if there is a side branch ostium that is being stented across, plaque just beyond the bifurcation moves and shifts into the side branch resulting in worsening of the narrowing in this vessel. Plaque shift is of greatest concern when the plaque is located on the carina or the apex of the bifurcation.
Alternatively, access into a jailed vessel can be attained by carefully placing a guide-wire through the stent, and subsequently tracking a balloon catheter through the stent struts. The balloon could then be expanded, thereby deforming the stent struts and forming an opening into the previously jailed vessel. The cell to be spread apart must be randomly and blindly selected by re-crossing the deployed stent with a guide-wire. The drawback with this approach is that there is no way to determine or guarantee that the main-vessel stent struts are properly oriented with respect to the side branch or that an appropriate stent cell has been selected by the wire for dilatation. The aperture created often does not provide a clear opening and creates a major distortion in the surrounding stent struts. A further drawback with this approach is that there is no way to tell if the main-vessel stent struts have been properly oriented and spread apart to provide a clear opening for stenting the side branch vessel. This technique also causes stent deformation to occur in the area adjacent to the carina, pulling the stent away from the vessel wall and partially obstructing flow in the originally non-jailed vessel. Deforming the stent struts to regain access into the previously jailed vessel is also a complicated and time consuming procedure associated with attendant risks to the patient and is typically performed only if considered an absolute necessity. The deformation of the contralateral struts has been addressed by doing a simultaneous inflation with two balloons, one being placed in the parent and the other in the daughter-vessel. The inability to place a guide-wire through the jailed lumen in a timely fashion could restrict blood supply and begin to precipitate symptoms of angina, resulting in myocardial infarction or even cardiac arrest.
Other methods employed include a “T-stent” procedure. This involves implanting a stent in the daughter-vessel ostium followed by stenting of the parent-vessel across the daughter-vessel. Subsequently deforming the struts as previously described, to allow blood flow and access into the daughter-vessel. Alternatively, a stent is deployed in the parent-vessel followed by subsequent strut deformation as previously described, and finally a stent is placed into the daughter-vessel. Stent deployment in the ostium of the daughter-vessel may be necessary if there is a significant plaque burden at the bifurcation and involves the ostium of the daughter-vessel. Conversely stenting of the daughter-vessel may be required to treat a possible dissection created by the initial angioplasty. T-stenting would theoretically be useful in situations where the angle between the parent and daughter vessels is 90-degrees. This is rarely the case in real life and the alignment of the stent in the daughter-vessel with the apex of the carina results in inadequate coverage of the ostium. Alignment of the stent to the beginning of the side branch ostium results in protrusion of stent struts into the parent-vessel lumen. Both scenarios increase the risk of subsequent complications and renarrowing.
In another prior art method for treating bifurcated vessels, commonly referred to as the “Culotte technique,” the side branch vessel is first stented so that the stent protrudes into the main or parent vessel. A dilatation is then performed in the main or parent vessel to open and stretch the stent struts extending across the lumen from the side branch vessel. Thereafter, a stent is implanted in the main branch so that its proximal end overlaps with the already-stented side branch vessel. One of the drawbacks of this approach is that the orientation of the stent elements protruding from the side branch vessel into the main vessel is completely random. In addition excessive metal coverage exists from overlapping strut elements in the parent vessel proximal to the carina area. Furthermore, the deployed stent must be recrossed with a wire and arbitrarily selecting a particular stent cell. When dilating the main vessel the stent struts are randomly stretched, thereby leaving the possibility of restricted access, incomplete lumen dilatation, and major stent distortion.
In another prior art procedure, known as “kissing” stents, a stent is implanted in the main vessel with a side branch stent partially extending into the main vessel creating a double-barrelled lumen of the two stents in the main vessel distal to the bifurcation. Another prior art approach includes a so-called “trouser legs and seat” approach, which includes implanting three stents, one stent in the side branch vessel, a second stent in a distal portion of the main vessel, and a third stent, or a proximal stent, in the main vessel just proximal to the bifurcation.
All of the above-mentioned examples of stent deployment techniques suffer from the same problems and limitations. There can be uncovered intimal surface segments on the daughter-vessel between the stented segment and the parent-vessel or there is excessive coverage in the parent vessel proximal to the bifurcation. An uncovered intimal surface with a possible dissection flap or uncompressed plaque will increase the risk for sub-acute thrombosis, and the increased risk of the development of restenosis. Further, where portions of the stent are left unapposed within the lumen, the risk for subacute thrombosis or the development of restenosis is increased also. The prior art stents and delivery assemblies for treating bifurcations are difficult to use and deliver making successful placement nearly impossible. Further, even where placement has been successful, the side branch vessel can be “jailed” or covered so that there is impaired access to the stented area for subsequent intervention.
Prior art Tryton bifurcation stent with trizone technology suffers from the same limitation of requiring to recross the stent struts of the parent-vessel stent with an arbitrary selection of a cell and subsequent deformation along with all its limitations as previously described.
One prior art stent that is specifically designed for bifurcations in the petal stent from Boston Scientific. This has a specifically designed collar which expands radially into the ostium of the daughter vessel. The collar is designed as radially placed struts covering the entire perimeter of the ostium. The symmetry of the collar is believed to be a drawback as most bifurcations have non-90° take off angles, and so the collar would create an unnecessary and excessive crowding or deformation of struts in the ostium.
The key element in the successful treatment of bifurcation with current art/technology is the simultaneous “kissing” balloon inflation. Successful placement of a guide-wire through the struts of the stent in the parent-vessel is essential for this to occur. Inability to cross with a guide-wire into the daughter-vessel would leave stent struts in the ostium and unopposed to the intimal surface. The present invention solves these and other problems as will be shown.